It is often desirable to guide radiated energy from a narrowband high power microwave source into a collimated microwave beam radiation pattern of variable diameter which can be directed to desired azimuth and elevation angles for ground or space-based applications. For the purposes of target location, a wide antenna beam is useful for acquiring a target quickly; however, the accuracy in determining a target's position is relatively low. Zoom capability enables an operator to focus in on the target once it is acquired by continuously decreasing the diameter of the collimated microwave beam and reacquiring the target to more accurately determine its position. What is needed are high-power capabilities for zoom antennas that can greatly increase the effective range of a high power microwave source and provide variable control over an area being illuminated at large distances and in a desired direction.
Some prior “zoom” antennas use reflectors to radiate conical antenna patterns, and broaden the beam by de-focusing it. These are not true zoom antennas and have a limited range due to rapid divergence of the beam. An antenna system consisting of confocal reflectors that creates a collimated microwave (or “pencil”) beam radiation pattern is proposed in U.S. Pat. No. 2,825,063, issued to Roy Spencer in 1958; however, the diameter of the pencil beam cannot be varied. Another drawback to the system described in the '063 patent is feed-blockage, which is a common drawback to many reflector antennas. Another zoom antenna concept proposing the use of reflectors and a multi-beam feed is disclosed in U.S. Pat. No. 3,938,162, issued to Richard Schmidt in 1976. However, the aforementioned system requires precise synchronization of the multiple beams, which is very difficult to achieve. According to the description in the '162 patent, the radiation pattern produced by this system was “severely distorted” and “unusable as a multibeam antenna.” The system also requires splitting the source energy into multiple beams and then recombining them, which makes this system very inefficient and therefore greatly reduces its effective range.
True zoom antennas that can produce a collimated beam with a variable diameter using reflectors are shown in U.S. Pat. No. 6,414,646, issued to Howard Luh in 2002, and also in the '162 patent. The concept proposed in the '162 patent consists of two parabolic reflectors with telescoping sections to vary their respective focal lengths. Zoom capability is achieved by incorporating telescoping sections into the reflectors to vary the respective shapes of the parabolic reflectors and thereby vary their focal lengths, and then repositioning the reflectors relative to each other to make them again confocal in order to achieve a collimated beam radiation pattern with a new diameter. These systems nonetheless encounter feed blockage problems and require high precision manufacturing given their required reflective properties.
What is needed in the art is a high power microwave zoom antenna that is accurate and can provide true zoom capability, yet is less costly to manufacturer and easier to implement than current microwave antenna systems.